The present invention relates broadly to a lock-in test or sensing amplifier, and more specifically to a lock-in amplifier including a switching arrangement connected between an input terminal and the inputs of two signal channels and which can be synchronized with the information component of an input signal supplied to the input terminal, a signal combining circuit coupled between the two signal channels and an output terminal, and a switch control device connected between the synchronizing signal input terminal and a switching device.
For the selective amplification of an information carrying signal having a given frequency and onto which an interfering signal of a different frequency and/or stochastic noises are superimposed, frequently lock-in amplifiers are used. In such known amplifiers the input signal which is to be processed and which contains the information carrying signal and the noises, is correlated with a reference signal, which has the same frequency as the information carrying signal. The correlation can be accomplished with a simple switching arrangement or with a mixer or with a multiplying circuit. The signal which is obtained after the correlation must be thereafter passed through a low-pass filter in order to free it from the reference frequency and also must be smoothed-out or filtered. The resulting means value signal is still dependent from the sampling ratio of the reference signal. The largest signal-to-noise ratio is obtained at a sampling ratio of 1:1, that is with a sinusoidal reference signal.
Many measurements result in an electrical signal in which the sections containing the useful information periodically alternate with sections which contain the reference information. Frequently, a similar time period for the segments containing the useful information and for the segments containing the reference information cannot be had, that is, the sampling ratio 1:1 is impossible or not realistic. This is the case when a pulsed process requires time for the regeneration or in the opposite case, when the flow of information of a process should be measured possibly in a continuous fashion, and the zero point must be checked only occasionally. For such purpose the known lock-in amplifiers are not satisfactory.
When the operation of the switching process causes a temporary cross-talk or non-linear distortions or interferences, such cannot be taken care of by the known lock-in amplifiers in an optimum fashion.